1. Field of the Invention
The present invention relates to a process for making fine particulate, non-agglomerating chloropolyethylene.
2. Description of the Prior Art
Chloropolyethylene (briefly termed CPE hereinafter) finds commercial use in the production of thermoplastic compositions; this has been fully described in German Patent Nos. 12 36 774; 12 66 969; 14 69 990 and 23 42 983; and in DE-OS No. 24 56 278. CPE is normally made by chlorinating low pressure polyethylene (briefly termed PE hereinafter) in aqueous phase. Needless to say, the chlorination results in the physical properties of the polyethylene being considerably modified. In the form of fine particulate powder, polyethylene has no distinct elastic properties, nor do its particles tend to agglomerate. As a result of its properties being modified, CPE on the other hand combines a rubber-like elasticity, which incidentally is highly desirable, with the undesirable tendency of its particles to undergo agglomeration. This is a problem which has been fully discussed e.g. in DE-AS No. 14 20 407 relating to the chlorination of PE having a linear molecular structure and a crystalline structure, and consisting of particles with a size between 0.1 and 300 microns.
In order to inhibit the agglomeration of particles during the chlorination of polyethylene in aqueous phase, it has been suggested in DE-PS No. 14 20 407 that this operation should be effected in the presence of inert addends. The addends specified are selected from silicic acid or kieselguhr, barium sulfate, powdered asbestos, titanium dioxide, graphite, silicon carbide, glass powder and halogenated plastics materials, such e.g. as polyvinyl chloride. By the operation just described, it is indeed possible to limit the agglomeration of CPE particles to a given extent, but it is impossible to retain in the CPE the particle size distribution initially found in PE. Technically adverse effects which originate from this fact have been discussed in German Patent Specification Nos. 22 60 525 and 23 43 983. To solve the problem, it has been suggested that the PE should be chlorinated with chlorine in aqueous phase in the presence of 0.1 to 2 weight % silicic acid and 0.01 up to 1 weight % silicon oil, based on the PE used. In the event of the PE being chlorinated as described in German Patent Specification No. 22 60 525 in the presence of silicic acid having the quality specified and in the presence of silicon oil, which preferably is a polysiloxane selected from the dimethylpolysiloxane series, it is actually possible for the particle-size distribution initially determined in PE to be retained also in the chlorinated product.
All these processing steps do, however, not permit the agglomeration of particles to be effectively inhibited during the work-up of the chlorinated mixture, i.e. during the separation of the solid matter from the suspension, during the washing out of hydrogen chloride adhering to the CPE, during the drying operation and especially during storage of the product. Needless to say, is is not allowable for any step which is taken in an attempt to avoid the agglomeration of particles to impair the quality of the final product or render its work-up difficult.
These are problems which have been referred to in German Patent Specification No. 23 43 983. It is more specifically described therein that the materials suggested in DE-AS No. 14 20 407 for use as addends inhibiting agglomeration, e.g. silicic acid or kieselguhr, on the one hand actually produce satisfactory results. On the other hand, however, they seriously affect the quality of the final product and also the rheological and mechanical properties of a blend made from CPE and PVC. In addition to this, it should be borne in mind that particle agglomeration also depends on the molecular weight of the PE used.